FIG. 1 of the accompanying drawings illustrates the stack structure of a typical liquid crystal display (LCD) module. The display 1 comprises a flat transmissive spatial light modulator (SLM) in the form of an LCD panel having input and output polarisers on its bottom and top sides. The rest of the structure is generally regarded as the backlight system, as follows. A light source 2 emits light, which is coupled into a light guide 3 and distributed across the back of the display 1 by way of total internal reflection (TIR) in such a way that if no scattering structures were present the light would travel until it reached the end of the light guide. Furthermore, the top and bottom major surfaces of the light guide are generally smooth and flat such that the average direction of the light within the light guide is not deviated from its initial average direction of propagation. However, within the light guide there are multiple scattering structures 4 that extract the light from the light guide to illuminate the LCD panel by disrupting the TIR conditions at the surface of the light guide on which they are located 5, hence allowing the light to pass through the air light guide interface. These scattering features may be located on either the top or bottom major light guide surfaces. The density of the light scattering features may increase with distance from the light source to maintain a uniform rate of extraction of the light along the length of the light guide. As light is extracted both down and up from the light guide, a reflecting film 6 is placed beneath the light guide to improve the efficiency of the backlight. There are also some optical films 7 between the light guide and the LCD panel, placed to give better illumination uniformity over the display area and to enhance brightness within a given viewing angle range. The nature of these films is well known and will not be described further here.
It would be useful at this point to define an item of terminology. The average direction of the light is defined as a single vector which indicates the sum of all the light direction vectors within the light guide. This may also apply to the light within a smaller region of a light guide. In a rectangular light guide that is evenly illuminated from one end, the average direction of light is directly away from the light source, perpendicular to the plane of the input face. This is the same regardless of light source, as long as the light source is located at one end of the light guide only. The average direction may also be discussed in terms of the direction of light within a smaller area than the full light guide.
Some conventional devices utilize grooves on one or more major surfaces of a light guide to allow for improved uniformity of the extracted light. For example:
In EP 1 876 480 A1 (Samsung) grooves are arrayed on one or both sides of the light guide. The grooves on the bottom are aligned parallel to the light direction. The grooves on the top are aligned perpendicular to the light direction and are used as extraction features. The upper ones are non-uniform in size and apex angle. The non-uniformity in groove dimension helps maintain brightness uniformity of the extracted light. The lower grooves do not alter size.
In EP 1 876 482 A1 (Samsung) uniform grooves may be arrayed on one or both sides of a light guide. The grooves can be parallel to the average light direction or may be arranged substantially perpendicular to the light direction to act as extraction features. Point features are added to the extraction grooves to aid in diffusion of the light towards an overlying display.
WO 02/04858 A2 (3M) describes grooves on one or both major surfaces of a light guide. The grooves on the lower surface are parallel to the light direction however, they are non-uniform. They may change their direction or linear structure to maintain the uniformity of extraction. The cross section may be triangular, arc or flat. The reference also describes a similar structured but arranged as a Fresnel-like lens.
US 2006/0291247 A1 (AU Optronics) describes grooves on one or both sides of a light guide. The grooves on the top surface are not designed as extraction features. Rather, having a constant cross section but a position that changes to create wave-like features the groove structure is designed to prevent Moiré interference between the light guide structure and the liquid crystal panel.
In EP 1 906 218 A1 (Samsung) grooves are arrayed on both top and bottom surfaces of a light guide. On the top surface the grooves are aligned parallel to the light direction. On the bottom surface there are two sets of orthogonal grooves, one set parallel to the light, the other set perpendicular to the light direction. The two sets of grooves intersect to create extraction regions whilst still maintaining good uniformity of the light distribution within the light guide.
In WO 2006/080710 A1 (Cheil Industries Inc.) sets of grooves on both the top and bottom major surfaces of the light guide are used. The top set of grooves is aligned parallel to the light direction. The bottom sets of grooves are arranged in discrete patches so that extraction is not achieved at all points of the bottom surface. Whilst the general alignment of the grooves within the patches is to be perpendicular to the light direction, some patches may have an alternate orientation. This is to further improve uniformity of the light extracted from the light guide.
Some conventional devices utilize grooves on the input, or light receiving, surface of a light guide to widen the input angular distribution and hence improve the uniformity. For example:
US 2004/0246601 A1 (Citizen Electronics Co Ltd) describes the use of grooves aligned vertically on the light receiving surface of a light guide to widen the input angular distribution of the light. Two sets of interlaced grooves with different apex angles are described to give two distinct input profiles, allowing a wider angular distribution of the light within the light guide.
Other, similar devices, are described in U.S. Pat. Nos. 7,067,753 B1, 7,325,958 B2 and JP 2004327096A.
U.S. Pat. No. 7,156,548 B2 (Innolux Display) describes a light guide that may have grooves on one or both major surfaces which are arranged as extraction features. The grooves are arrayed nominally perpendicular to the direction of the light within the light guide. The depth, separation and apex angle of the grooves control the strength of the extraction and the relative angle of the surfaces of the grooves the direction of the extracted light.
US 2004/0246697 A1 (Yamashita) describes grooves cut into one surface of the light guide. They may have a prism or lens structure. The grooves are designed to enhance the uniformity of the light guide but not to extract the light itself. They are aligned nominally parallel to the direction of the light.
Current LCD utilization is in the form of a rectangular screen the under side of which is illuminated by the backlight arrangement described above. Such a shape of display is provided with uniform light which has been distributed by TIR within the light guide. However, a display, or general lighting illumination panel, may need to be designed to have a shape other than a rectangle or square. These could include, by way of example, light guides that trace an in-plane turn, light guides with intrusive cuts made into the minor side surfaces or light guides with a hole made in the middle, from bottom to top major surfaces. Examples of the first two shapes may be an L-shaped light guide 8 or an hour-glass shaped light guide 9 respectively, as demonstrated in FIGS. 2a and 2b. Possible positions for the light sources 2 and resulting direction of light travel 10 within these light guides are shown. The possible shapes of light guide and number and position of light sources should not be limited to these examples.
In these instances it is conceivable that there will be a significant shadow region caused, whereby light emitted from the light source has no direct optical path to a region in the lea of such a feature, denoted by 11 for both cases in FIGS. 2a and 2b. The definition of “direct optical path” should be clarified to mean a straight line that can be drawn from the location at which light is coupled into a light guide, from a light source, to the region of interest without intersecting a side wall of the light guide, thereby causing the line to leave the confines of the material of the light guide. Fundamental to this is the range of angles that the light may take on coupling into the light guide which is limited by the refractive indices of the light guide and the surrounding medium; this results in a cone inside of which light may propagate, but outside of which it may not. Therefore, a region of the light guide outside of this cone is deemed to not be on a direct optical path from the light source.
Reflection of light from the far end of the light guide 12 and from the side walls 13 will go some way to provide illumination to the shadow region, but there will still be significant reduction of brightness uniformity of the display in this shadow area. This issue may relate to both a light guide punctured by a hole 14 from top to bottom major surfaces and one with intrusions cut into the sides (the hour glass or L-shaped light guide examples of the latter). Additionally, designs incorporating intrusions or holes will suffer from significant losses of light at the obstruction, or intrusion, surface nearest the light source. This loss will occur as TIR requires that the incident angle of the light upon an interface from high to low refractive index is above a critical angle to the normal of the interface, defined by the arcsine of the ratio of the refractive indices; in this case those of the light guide and the surrounding medium. At smaller angles of incidence the light will be allowed to pass through the interface and be extracted from the light guide. The relative angle of the intrusion surface to that of the light path will no longer satisfy this criterion in the majority of cases and hence strong transmission of light will occur out of the light guide.
Providing minor side surfaces (such as those indicated by 13 in FIGS. 2a and 2b) that are reflective via the application of a layer of metal, for example, may aid in the redistribution of light within the light guide and mitigate transmission loses. However, reflection efficiency at these surfaces will never be perfect, unlike that occurring during TIR. Furthermore, the inclusion of a metal surface in the design will add another manufacture step and may itself cause a shadow region, dependent on the application method and position. The best method of prevention of light loss is to reduce the amount of light actually interacting with any intrusive surfaces.
The re-distribution of light within a light guide may be termed sub-guiding. It represents the distribution of light within a larger light guiding structure in such a way that is nominally independent of the larger shape of the light guide and the angular and positional distribution of LEDs.